for now revert type changes to avoid mypy errors, add validation for through models, clean docs etc

docs/releases.md

@@ -29,7 +29,8 @@
     * you can include and exclude fields on through models
     * through models are attached only to related models (i.e. if you query from A to B -> only on B)
     * note that through models are explicitly loaded without relations -> relation is already populated in ManyToMany field. 
-    * note that just like before you cannot declare the relation fields on through model, they will be populated for you by `ormar`
+    * note that just like before you cannot declare the relation fields on through model, they will be populated for you by `ormar`,
+      but now if you try to do so `ModelDefinitionError` will be thrown
     * check the updated ManyToMany relation docs for more information
 
 # Other

ormar/fields/many_to_many.py

@@ -3,6 +3,7 @@ from typing import Any, List, Optional, TYPE_CHECKING, Tuple, Type, Union, cast
 
 from pydantic.typing import ForwardRef, evaluate_forwardref
 import ormar  # noqa: I100
+from ormar import ModelDefinitionError
 from ormar.fields import BaseField
 from ormar.fields.foreign_key import ForeignKeyField
 
@@ -17,6 +18,21 @@ if TYPE_CHECKING:  # pragma no cover
 REF_PREFIX = "#/components/schemas/"
 
 
+def forbid_through_relations(through: Type["Model"]) -> None:
+    """
+    Verifies if the through model does not have relations.
+
+    :param through: through Model to be checked
+    :type through: Type['Model]
+    """
+    if any(field.is_relation for field in through.Meta.model_fields.values()):
+        raise ModelDefinitionError(
+            f"Through Models cannot have explicit relations "
+            f"defined. Remove the relations from Model "
+            f"{through.get_name(lower=False)}"
+        )
+
+
 def populate_m2m_params_based_on_to_model(
     to: Type["Model"], nullable: bool
 ) -> Tuple[Any, Any]:
@@ -77,6 +93,8 @@ def ManyToMany(
     nullable = kwargs.pop("nullable", True)
     owner = kwargs.pop("owner", None)
     self_reference = kwargs.pop("self_reference", False)
+    if through is not None and through.__class__ != ForwardRef:
+        forbid_through_relations(cast(Type["Model"], through))
 
     if to.__class__ == ForwardRef:
         __type__ = to if not nullable else Optional[to]
@@ -189,6 +207,7 @@ class ManyToManyField(ForeignKeyField, ormar.QuerySetProtocol, ormar.RelationPro
                 globalns,
                 localns or None,
             )
+            forbid_through_relations(cls.through)
 
     @classmethod
     def get_relation_name(cls) -> str:

tests/test_m2m_through_fields.py

@@ -375,25 +375,3 @@ async def test_excluding_fields_on_through_model() -> Any:
         for category in post3.categories:
             assert category.postcategory.param_name is None
             assert category.postcategory.sort_order is None
-
-
-# TODO: check/ modify following
-
-# add to fields with class lower name (V)
-# forward refs update (V)
-# creating while adding to relation (kwargs in add) (V)
-# creating in queryset proxy (dict with through name and kwargs) (V)
-# loading the data into model instance of though model (V) <- fix fields ane exclude
-# accessing from instance (V) <- no both sides only nested one is relevant, fix one side
-# filtering in filter (through name normally) (V) < - table prefix from normal relation,
-#               check if is_through needed, resolved side of relation
-# ordering by in order_by (V)
-# updating in query (V)
-# updating from querysetproxy (V)
-# including/excluding in fields? (V)
-# make through optional? auto-generated for cases other fields are missing? (V)
-
-# modifying from instance (both sides?) (X) <= no, the loaded one doesn't have relations
-# allowing to change fk fields names in through model? (X) <= separate issue
-
-# prevent adding relation on through field definition

tests/test_through_relations_fail.py

@@ -0,0 +1,51 @@
+# type: ignore
+
+import databases
+import pytest
+import sqlalchemy
+
+import ormar
+from ormar import ModelDefinitionError
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+def test_through_with_relation_fails():
+    class BaseMeta(ormar.ModelMeta):
+        database = database
+        metadata = metadata
+
+    class Category(ormar.Model):
+        class Meta(BaseMeta):
+            tablename = "categories"
+
+        id = ormar.Integer(primary_key=True)
+        name = ormar.String(max_length=40)
+
+    class Blog(ormar.Model):
+        class Meta(BaseMeta):
+            pass
+
+        id: int = ormar.Integer(primary_key=True)
+        title: str = ormar.String(max_length=200)
+
+    class PostCategory(ormar.Model):
+        class Meta(BaseMeta):
+            tablename = "posts_x_categories"
+
+        id: int = ormar.Integer(primary_key=True)
+        sort_order: int = ormar.Integer(nullable=True)
+        param_name: str = ormar.String(default="Name", max_length=200)
+        blog = ormar.ForeignKey(Blog)
+
+    with pytest.raises(ModelDefinitionError):
+
+        class Post(ormar.Model):
+            class Meta(BaseMeta):
+                pass
+
+            id: int = ormar.Integer(primary_key=True)
+            title: str = ormar.String(max_length=200)
+            categories = ormar.ManyToMany(Category, through=PostCategory)